1. Field of the Invention
This invention relates to an electrode system for measuring oxygen gas concentration in a solution, especially in blood.
Determination of oxygen gas in blood has been conducted in clinical fields including diagnosis of various diseases of circulatory and respiratory organs, the examination of endocrine, metabolic, and digestive organ diseases, nacotism, and the control of respiratory function after operations. Much attention has been paid to oxygen concentration in blood of babies born immature and infants in a syncopic state. When oxygen is lacking in the blood it does not reach all parts of the body resulting in death or suffering from cerebral palsy. Conversely, if oxygen in the blood is in great excess it is responsible for diseases like retinitis (in immature babies).
Therefore, it is very important for a person treating a newly born baby or a seriously injured patient requiring artificial respiration to determine the content of oxygen or its partial pressure in blood, especially in arterial blood, in order to control their respiration.
2. Description of the Prior Art
There have been a few disclosures of electrodes and systems for the electrochemical analysis of oxygen in a solution such as blood.
U.S. Pat. No. 2,913,386 discloses an electrolytic device called a "CLARK ELECTRODE" for use in chemical analysis, and particularly a polarographic cell adaptable for use in making quantitative analysis, especially continuous analysis. It comprises an electrode pair supported in a predetermined spaced relationship and electrically connected by an electrolyte or a substance reactable to form an electrolyte, and a selectively permeable barrier for separating the electrode pair and the electrolyte or other substance from the composition to be analyzed.
U.S. Pat. No. 3,372,103 discloses an improved dissolved oxygen probe in order to obtain a fast response, a stable reading and a long performance life. In this probe, the surface of a lead anode is large relative to that of a platinum cathode, at least 30 to 1. The surface area of the lead rod in square inches relative to the volume of electrolyte in milliliters is greater than one.
U.S. Pat. No. 3,088,905 discloses an electrode system which is not affected by solution movement and other constituents in the test solution, for example proteins in blood or metal ions in industrial solutions. The negative electrode consisting of a platinum or gold wire is fixed co-axially within a silver tube acting as the positive electrode by means of an electrically insulating material such as glass, filling the space between the wire and tube. The whole thus forms a solid rod, one end of which is ground flat.
Briefly, an electrode system conventionally used for measuring oxygen content of a fluid comprises an indicator electrode and a reference electrode which are immersed in an electrolyte solution and an oxygen permeable membrane against which the indicator electrode is pressed. A sample solution to be analyzed is located on the other side of the membrane. Since a voltage of 0.4 volt to 0.8 volt is applied between the two electrodes, oxygen penetrating through the membrane is electrolized, resulting in a flowing polarographic current proportional to the concentration of oxygen in the sample solution.
In this type of electrode system, Ag/AgCl or Ag is used as a reference electrode. Concerning the Ag/AgCl electrode, its surface turns into an silver chloride rich electrode through electrolysis because of the dissolving of silver. On the other hand, silver deposits on the surface of the indicator electrode. When the amount of oxygen in some blood which is in contact with the membrane is determined, the coagulation of blood sometimes occurs on the surface of the membrane resulting in retardation of the response of a reading.